In steam turbine construction, for example, curved guide blades are used as an embodiment of turbine blades especially when high three-dimensional flows occur which exhibit pronounced radial differences in the static pressure profile between the rotor side and the stator side, these differences arising due to deflection in the guide blades. In steam turbines, especially in low-pressure turbines with a large outflow cross section, the blade length to hub ratio is relatively high. The flow of a flow medium in the last stage of a low-pressure turbine having a large inflow cross section leads, in the case of a high blade length to hub ratio, to a radial reaction distribution which has an adverse effect on the efficiency of the steam turbine. The reaction distribution is in this case different in the radial direction and is low at the hub and high at the casing, this being felt to be a disadvantage.
In a thermal turbomachine, the percentage fraction of the isentropic enthalpy gradient in moving blades in relation to the entire isentropic enthalpy gradient over a stage consisting of a guide blade ring and a moving blade ring is designated as the isentropic reaction degree r. Such a stage in which the reaction degree is r=0 and the highest enthalpy gradient occurs is designated as a straightforward constant-pressure stage.
In a conventional excess-pressure stage, the reaction degree is r=0.5, so that the enthalpy gradient in the guide blades is exactly the same as in the moving blades. A reaction degree of r=0.75 is designated as a strong reaction. In steam turbine construction practice, the conventional excess-pressure stage and the constant-pressure stage are predominantly employed. As a rule, however, the latter has a reaction degree somewhat different from zero.
A low or even negative reaction of the hub leads to severe impairments and to efficiency losses of the turbine during operation. A high reaction of the casing gives rise to a high attack velocity of the moving blades in the tip region. The high attack velocity has an adverse effect on efficiency, since the behavior of flow losses is squarely proportional to velocity. A reduction in the reaction would remedy this. Moreover, a lower reaction of the casing would lead to a reduction in the gap losses, and the efficiency would thereby be additionally improved.
A high reaction in the hub region reduces the gap losses in the guide blade ring and thus leads to improved efficiency.
Curved guide blades are in this case used, in particular, in order to optimize the radial reaction distribution.
Turbines with guide blades curved only in the circumferential direction are known, for example, from DE 37 43 738. This shows and describes blades, the curvature of which is directed over the blade height toward the delivery side of the guide blade in each case adjacent to the circumferential direction. This publication also discloses blades, the curvature of which is directed over the blade height toward the suction side of the guide blade in each case adjacent to the circumferential direction.
Consequently, both radial and circumferentially running boundary layer pressure gradients are to be effectively reduced, and consequently the aerodynamic blade losses are to decrease in size.
Turbines with guide blades curved in the direction of flow and in the circumferential direction are known, for example, from DE 42 28 879.
Curved guide blades are also known from U.S. Pat. No. 6,099,248.